Why does down need to breathe?

[wildwanderer]

One thing that has always puzzled me. We are told to store down sleeping bags in cotton bags so all the down to remains uncompressed and air can circulate freely.

I can understand keeping the down uncompressed. But why is a cotton bag suggested? Instead of a large plastic bag or garbage bag etc?

Why does the down need to breathe? does it prevent moisture?

Im trying to avoid the need to sew cloth bags for my down gear.

[Warin]

Number of thoughts - no proof...

When first put away it is probably a little moist from body sweat .. good to let that breath out.

When the temperature changes it could cause moisture to condense out into water .. not good .. best to let it breath out.

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Source of cotton should not be too hard .. new bed sheets and use the old sheets to make the bags. And it is good practice sewing as the result is seldom seen.

[Mark F]

Breathing is a bad term, down doesn't breathe. What it is trying to convey is that air and any excess water vapour needs to be able to pass out of the down or synthetic fill product to minimise the risk of moulds (mildew) developing. If the down is heavily compressed, or the bag is in a waterproof/resistant stuff sack, it will inhibit this process. So store any insulation filled item by loosely packing it in a highly breathable stuff sack (cotton or mesh work well) or hanging it in your wardrobe. As Warin says, old bed sheets are good and calico is cheaply available from fabric shops.

[wildwanderer]

Do you reckon a plastic bag with the top partially open and a couple of air holes would suffice?

[Warin]

Do you reckon a plastic bag with the top partially open and a couple of air holes would suffice?

No.
flee bay search on 'cotton bag' got good results - $5.35 32 x 38 cm Should last longer than the plastic bag too.

[trekker76]

Do you reckon a plastic bag with the top partially open and a couple of air holes would suffice?

Id say no. Here where mould is terrible there is a difference between free hanging with airflow, mesh bags, solid cloth bags and plastic/synthetic bags, in descending order of usefulness. Wont be as bad for most climates but you can apply the above as a comparison.

[Huntsman247]

Most down bags come with storage sacks don't they?

[wayno]

your body gives off a lot of moisture overnight, esp when its warm..... over time thats litres of water that end up going through the bag,
unless you're in a hot bone dry environment, a fair bit of moisture remains in the bag,
use it in a damp environment, esp when its cold, and esp when the dew point is inside the outer shell of the bag.. and see how fast the bag looses its loft, because the moisture builds up...
in an environment like NZ, your bag is almost always pretty damp after a trip.
you have water repellent treated down now, which helps but it still needs to be aired....